1. Field of the Invention
The present invention relates to a fuel assembly for a nuclear reactor using a coolant such as a liquid metal, and particularly to a fuel assembly which is configured to store and hold a plurality of fuel pins in a wrapper tube by using grids and liner tubes and which suppresses an unnecessary flow of the coolant in an outer circumferential side in the wrapper tube and increases the flow volume of the coolant passing through interiorly disposed ones of the fuel pins to thereby increase the core power.
2. Related Art
Generally, in a nuclear reactor, a fuel assembly is supported in a reactor core while being attached to a support member. In a nuclear reactor using a coolant such as a liquid metal, an electromagnetic pump is used as a drive source to circulate the coolant around a plurality of fuel pins included in the fuel assembly supported in the reactor core. In this case, if the nuclear reactor is small-sized, the fuel assembly is configured to store the fuel pins in a wrapper tube to enable the circulation of the coolant with no need for the drive source. The wrapper tube is configured to include an entrance nozzle at a lower end thereof for introducing the coolant, and an operation handling head at an upper end thereof. The wrapper tube includes therein grids for supporting the fuel pins in the radial direction of the wrapper tube, and liner tubes inserted in the wrapper tube for fixedly holding the respective grids in the axial direction of the wrapper tube. The intervals in the radial direction of the fuel pins are kept by the grids. Meanwhile, the intervals in the axial direction of the grids are kept by a tie rod, the liner tubes, or the like (see Japanese Unexamined Patent Application Publication No. 6-174882, for example).
FIGS. 23 and 24 illustrate the configuration of this type of conventional fuel assembly. In the figures, a plurality of fuel pins 101 are stored in a wrapper tube 103, with the pin intervals of the fuel pins 101 being kept by grids 102. Each of the fuel pins 101 is fixed at a lower portion thereof by a lower pin support plate 105 and at an upper portion thereof by an upper pin support plate 106. The coolant such as a liquid metal flows in from a coolant inlet 108 of an entrance nozzle 104 and flows out from a coolant outlet 109 of a handling head 107.
In the thus configured fuel assembly, as illustrated in FIG. 25, each of the grids 102, which has a low pressure drop, includes a grid frame 102a provided with a multitude of ring-shaped pin support members 110. As illustrated in FIG. 26, for example, each of the pin support members 110 is provided with three dimples 110a on the inside thereof such that the circumference of the corresponding fuel pin 101 is three-point supported, for example, by the dimples 110a. 
FIG. 27 illustrates a deformation state in which the wrapper tube 103 is expanded by the thermal expansion. That is, the wrapper tube 103, the basic form of which is a regular hexagon as indicated by a virtual line in FIG. 27, is expanded when used due to the irradiation deformation and is deformed so as to expand toward the outer circumference thereof as indicated by a solid line. Conventionally, to cope with such deformation, liner tubes each formed by a thin hexagonal tube are provided outside a fuel bundle such that the liner tubes and the grids are alternately stacked. Thereby, the intervals in the axial direction of the grids are kept.
In such a configuration, however, the flow passage area around the fuel bundle is large. Thus, the cladding temperature in a central area of the fuel bundle becomes relatively high in some cases. Therefore, there arises a need to keep the cladding temperature equal to or lower than a cladding temperature limit. As a result, the thermal efficiency is decreased.
To address this issue, the inventors of the subject application have proposed a technique for reducing the cladding temperature, in which followers each having a triangular cross section are provided to reduce the flow passage area of a bundle edge sub-channel in a core heat generation unit for preventing a peripheral flow. That is, according to the technique, the liner tubes are provided with peripheral flow preventing projections to suppress the occurrence of the above-described phenomenon (see “Development of Densely. Packed and Low-Pressure-Drop Fuel Assembly for Non-Refueling Core (3),” 2004 Fall Meeting Preliminary Proceedings 307 of the Atomic Energy Society of Japan, for example).
Meanwhile, in the above-described conventional configuration, the liner tubes and the grids are stacked and may be mutually misaligned in the radial direction. If the liner tubes and the grids are misaligned in the radial direction, an opening may be formed between the wrapper tube and the liner tubes to allow the coolant to flow from inside the liner tubes into the space on the wrapper tube side as a waste flow.
Further, in the conventional fuel assembly, the bulging deformation occurs in the wrapper tube by the irradiation creep due to the inner pressure of the wrapper tube. It is therefore possible in the expanded portion that the flow passage area of a peripheral region around the fuel bundle is increased while the flow volume of the coolant in the central area of the fuel bundle is reduced, and thus that the cladding temperature is increased. It is also possible that the liner tubes are similarly expanded due to the inner pressure applied thereto.